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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
     #userSpan{
            color: #999;
            font-size: 12px;
        }
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</head>
<body>
    <span>这是一个非常干净的标签</span>
    <br>
    用户名: <input type="text"> <span id="userSpan">*用户的格式</span>
</body>
</html>